In the fabrication of semiconductor integrated circuits optical pattern transfer tools are used to produce an optical pattern on photoresist which is a photosensitive material which is generally baked subsequent to its deposition on a semiconductor substrate structure. Light is directed at the optical pattern transfer tool. Transparent portions of the optical pattern transfer tool transmit the light to selected portions of the photoresist while opaque portions of the optical pattern transfer tool prohibit the light from reaching the remaining portions of the photoresist.
When negative resist is used, photoresist not exposed to light is removed to expose portions of the substrate structure while the remaining resist protects unexposed portions of the substrate structure during subsequent processing steps. The opposite is true when positive resist is used. In this case the photo resist exposed to the light is removed. During a development step the portions to be removed are usually rendered soluble in a base solution and rinsed from the semiconductor substrate structure in de-ionized (DI) water. Remaining resist may then be hardened by re-baking to ensure adhesion of the resist to the semiconductor substrate structure.
There are different optical configurations available to expose the optical pattern transfer tool. It should be noted that presently there are "two sizes" of optical pattern transfer tools, a mask and a reticle. The mask is used to pattern an entire semiconductor wafer in one exposure step. In this case the pattern for the entire wafer is represented on the optical pattern transfer tool. The reticle represents a pattern for only a portion of the entire wafer and is stepped across the wafer during exposure to light in order to pattern the entire wafer.
There are at least two important criteria of an optical pattern transfer tool which concern the engineer. First the engineer wants to be able to determine an exact size, within tolerances, of a feature having a critical dimension. Second the engineer wants to be able to define an area having a smallest width possible without being concerned with its exact dimension.
One present optical pattern transfer tool comprises two types of materials, one which transmits light, such as quartz, and one which prohibits the transfer of light, such as chrome. Using methods presently available it is possible to create a chrome region having a critical dimension falling within specified tolerances. Thus, there is a minimum width the chrome can be and still have its exact size fall within the critical dimension specification. However, using this "two material" optical pattern transfer tool does not allow definition of very small regions on the photoresist due to the limitations of photolithography.
A "one material" optical pattern transfer tool comprises a transparent material having two surfaces, where each surface lies in a plane which is parallel to a plane of the other surface. Between the planes "steps" are formed. Each step is formed where the material transitions from one planar surface to the other planar surface. Optical transfer of light is substantially prohibited at the step. This results in very small areas of the photoresist not being exposed to light, which allows definition of very small regions on the photoresist. However, using the "one material" optical pattern transfer tool does not allow dimensions to be controlled with a high degree of accuracy.
Furthermore, memory density is typically limited by a minimum lithographic feature size (F) that is imposed by lithographic processes used during fabrication. For example, the present generation of high density dynamic random access memories (DRAMs), which are capable of storing 256 Megabits of data, require an area of 8F.sup.2 per bit of data. There is a need in the art to maximize active areas on a memory device as compared to field areas in order to support increased storage capacity.
Accordingly, wordlines in these higher density memory devices require minimal width and must be accurately defined to stay within critical dimensions. When small dimensions are defined on a DRAM and critical dimensions are controlled, increased memory density is supported. Therefore, there is a need for an optical transfer tool in the manufacture of semiconductor devices wherein very small regions on a photoresist can be defined and these regions can be controlled with a high degree of accuracy.